Most tanker trucks and railroad tanker cars, which might contain any sort of liquid such as oil, chemicals, milk, water, and so on, typically have an access hatch at the top of the tank. Liquids are loaded into the tank through this access hatch by way of a large capacity discharge spout mounted on a raised tanker truck access platform structure, which is typically in the form of a large metal frame comprising a tanker access platform accessed by way of stairs. The access platform has an extension portion that extends outwardly therefrom so as to provide access to the access hatch at the top of the tanker truck. Further, access may be gained to the interior of the tank through this access hatch for purposes of cleaning, inspection, repair work, and so on.
In order to engage the loading spout into the access hatch, it is necessary that a person physically be at the access hatch on top of the tanker truck in order to open and close the access hatch and to manipulate and operate the discharge spout. Typically, a person, usually the truck driver, must climb up onto the tanker truck access platform structure by way of the stairs, and then onto the access platform. In early prior art access platform structures, a pivotally mounted end walkway of the access platform extended to the area of the tanker truck access hatch. The end walkway is pivotally moved between a raised position and a lowered position. In the raised position, the extension portion of the access platform would be out of the way of the tanker truck so as to preclude a tanker truck from potentially driving into the access platform. In the lowered position, the extension portion of the access platform would provide access to the access hatch on the top of the tanker truck. Typically, the access platforms on early prior art tanker truck access platform structures had a single set lowered position of a non-adjustable fixed height. Tanker trucks and railroad tank cars are of various heights, however, and vary in height by several feet. This variation of several feet in height of tanker trucks and railroad tank cars combined with the non-adjustable fixed height of access platforms can be quite dangerous. A person, such as a truck driver, may have to jump up or down, perhaps two or three feet, or even more, from the access platform to the tanker truck. This jumping up or down may be especially dangerous in winter weather conditions.
A more recently developed type of access platform system is one that has an access platform that remains level and moves vertically in a slidable sense to a selected height so as to be vertically aligned with the access hatch at the top of a tanker truck or railroad tank car. While this system works well in principle, there are specific problems associated with prior art tanker access platform systems employing this type of vertically slidable access platform. Typically, such vertically slidable access platforms are driven by hydraulics. The disadvantage of this type of system is that there is no "give" in the system--that is to say that the access platform does not yield to surrounding objects as it moves. It is a common occurrence to have a platform being lowered onto a tanker truck and to not stop at the appropriate height, but instead to forcibly contact the tanker truck, thus potentially causing damage to either the tanker truck or the access platform, which is obviously quite undesirable. While it is possible to employ limit switches or the like to stop the access platform at the appropriate height, this adds needless expense and complication to the tanker access platform system.
An improved type of tanker access platform system employs either belts or cables to raise and lower the access platform and is typically powered by either a hand operated wheel or a powered motor such as an electric motor. An advantage of a tanker access platform drive system that uses a belt or cable is that the belt or cable can "give" so as to preclude the access platform from forcibly lowering onto the tanker truck or tanker access platform. It is very important that in this type of access platform drive system that the cable, pulleys, and the moving platform itself, be aligned very accurately for various reasons, such as buckling of the drive belt or slipping of the cable on a pulley. It has been found, however, that misalignment, and therefor subsequent binding, of the access platform and of the drive mechanism in general can occur quite readily.